The drilling of oil and gas wells by the rotary technique involves the circulation of a drilling fluid through the drill string and out the bit nozzles and its return to the surface via the annulus. This fluid cools and lubricates the drill string, develops sufficient hydrostatic head to counterbalance formation pressures, and removes the cuttings from the borehole. This fluid also helps reduce the frictional forces between the drill string and the borehole or casing.
During the drilling operation the drill string may become stuck and cannot be raised, lowered, or rotated. There are a number of mechanisms possible which may contribute to this problem. Namely these are (1) cuttings or slough build-up in the hole; (2) an undergage hole; (3) key-seating; and, (4) differential pressures.
Differential sticking may be defined as the sticking of the drill string against a permeable formation containing less pore fluid pressure than the hydrostatic pressure exerted by the drilling fluid column and usually occurs when the drill string remains motionless for a period of time. The mechanism by which this occurs involves the drill string coming into contact with a mud filter cake built up against a permeable zone, remaining quiescent for a period of time sufficient for the drill pipe to be pressed into the mud cake by excessive pressure in the borehole, and thus being sealed against the wall of the borehole. The annular pressure exerted by the drilling fluid then holds the pipe against the borehole wall in the permeable zone.
Freeing of differentially stuck pipe is essentially a matter of reducing this pressure differential which exists across the pipe. One method used simply involves a reduction in fluid pressure by replacing the annular fluid with a less dense fluid such as diesel or crude oil allowing for less pressure differential to exist between the borehole and formation. In some cases the formation pressure may exceed the annular pressure which in turn allows the pipe to be blown away from the borehole wall. Oftentimes, it is unsafe to perform this operation since there is a high likelihood of a harmful blow-out of the well.
Release of stuck drill pipe is achieved conventionally by the placement in and by movement through the circulating mud system of a volume of a release agent sufficient to fully contact the region of the borehole where the pipe is stuck. This method is colloquially known as "spotting a pill". The most commonly used current method to release stuck pipe is the spotting of a pill of a suitably densified, surfactant fortified oil mud in the hole opposite the stuck interval. With time, the integrity of the mud filter cake between the drill pipe and borehole is reduced by the oil mud allowing pressure equalization on all sides of the pipe. Also, oil invasion into the mud cake reduces the adhesive forces and lubricates the area between the pipe and borehole resulting in less friction and quicker release.
In recent years, there have been a number of oil-based proprietary formulations developed which are aimed at releasing differentially stuck pipe, which formulations include: petroleum oil containing a material selected from the group consisting of 2-heptadecenyl-4-4-dimethoxy-2-oxazoline; 2-heptadecenyl-4-methoxy-4-methyl-2-oxazoline; the sodium salt of dioctyl sulfosuccinate; a mixture of a product containing high molecular weight free fatty acids, esters and alcohols made by acid cracking wool grease with about one-fourth its weight of the sodium salt of sulfated oleyl alcohol and mixtures thereof in a quantity sufficient to reduce interfacial tension at an interface between the petroleum oil and water to not more than 2 dynes per centimeter (U.S. Pat. No. 3,217,802); and an oil-based spotting fluid which contains propoxylated C.sub.18 -C.sub.32 alkanols, an oil-soluble emulsifier-wetting agent and if desired an imidazoline for high temperature emulsion stability (U.S. Pat. Nos. 4,436,638 and 4,464,269). Environmental concerns enforced by world-wide government regulations are increasingly limiting the application of oil-based formulations to drilling operations.
The patent literature reports of water-based formulations for the releasing of differentially stuck pipe include: a surface-active agent prepared from about four parts of half ammonium, half isopropylamine salt of the sulfosuccinic acid ester of the oleic acid amide of isopropanol amine and about one part of the isopropylamine salt of dodecylbenzenesulfonic acid which agent is added to the water base drilling fluid (U.S. Pat. No. 3,233,622); a polyethylene glycol having a molecular weight ranging from 106 to about 600 or a saturated salt water solution or a sea water solution of said glycol (U.S. Pat. No. 4,230,587); and an additive composition which comprises a polymer such as hydroxyethylcellulose, carboxymethylcellulose and mixtures thereof in brine or polyethylene oxide, ethylene oxide-propylene oxide copolymer, poly(vinylmethylether) and mixtures thereof in water or brine (U.S. Pat. No. 4,466,486).
There remains a need for improved spotting pills with low toxicity and, particularly, for effective water miscible chemical systems which are drilling fluid compatible and can better release stuck drill pipes, especially those which have improved efficacy for releasing differentially stuck pipe.